Coaptive electrothermal tissue fusion or sealing involves the application of force and electrical energy to heat compressed tissue sufficiently to join together separate pieces of tissue. Electrothermal tissue fusion avoids the need to manually suture or tie-off tissues or vessels during a surgical procedure.
Although the exact details of the physical chemistry involved in tissue fusion are probably not completely understood, it is believed that the heat denatures chains or strands of tissue proteins in the separate pieces of tissue and the pressure causes the denatured protein chains to reconstitute or re-nature across the interface between the tissue pieces. The reconstituted proteins chains interact and intertwine with one another to hold the previously-separate tissues pieces together.
Collagen is one type of protein chain that appears to play an important role in tissue fusion. Collagen, also known as tropocollagen, consists of three polypeptide protein chains that form a triple helix. These protein chains are grouped or tangled together to establish significant tissue structure and strength, as is observed in blood vessels and ligaments. Applying heat to the tissue to raise the temperature to about 60-70° C. causes the protein chains to become disordered, disassociated, separated and untangled from the triple helix.
Elastin is another type of protein chain that appears to play an important role in tissue fusion. Elastin a collection of polypeptide protein chains that are individually and randomly cross-linked with each other to form a fibril. Fibrils are grouped or tangled together to form an elastin fiber. Upon the application of heat to raise the temperature to about 120° C., the elastin fiber becomes disassociated into a disordered collection of individual polypeptide chains, fibrils and fibers.
The heat which causes denaturation of the collagen and elastin chains also appears to create unfavorable molecular interactions among the components of the denatured proteins, resulting in a relatively high free energy state. Atoms with the same electrostatic charge, and hydrophobic and hydrophillic regions of the protein chains, begin to interact and create repulsive forces. Force must be applied at the interface between the tissue pieces during fusion to overcome the repulsive forces and to achieve more favorable interactions of the proteins chains thereby reducing the amount of free energy. Force must also be applied at the interface to maintain the denatured protein chains in physical proximity with each other so that they will reconstitute and join the tissue pieces together.
Although this theoretical model of tissue fusion is understandable, reliable tissue fusion is difficult to achieve on a consistent basis. Fusing blood vessels is of particular concern, because vessel fusion during a surgical procedure is the primary use of tissue fusion at the present time. Fused blood vessels that fail or leak after the conclusion of surgery lead to internal bleeding. Internal bleeding usually requires a second operation to gain access to and seal the leaking vessel, which induces further trauma and risk to the patient.
One prior art type of electrosurgical tissue fusion involves bipolar electrosurgery. The tissues are compressed between two jaws of a forceps-type instrument. The jaws also serve as electrodes to conduct high-voltage radio frequency (RF) current through the compressed tissue. Heat is generated from the RF current flowing through the resistance or impedance of the tissue, and that heat denatures the chains of protein.
Certain difficulties arise when using bipolar electrosurgical tissue fusion. The voltage between the jaws which compress the tissue and serve as electrodes is typically several thousand volts. The distance between the jaws is relatively small when the tissue is compressed. The relatively high voltage can create arcs which jump the small distance between the jaws and penetrate the tissue adjacent to the jaws, particularly toward the end of the fusion procedure when the tissue between the jaws dehydrates and its impedance increases. The arcs enter the tissue in minuscule spots and destroy or weaken the tissue at those spots. Under conditions of prolonged application of RF power in this manner, which is typical with bipolar electrosurgical tissue fusion, the arcing can actually perforate the tissue adjacent to the fused area, thereby rupturing the tissue and destroying any sealing effect from the sealed area if there are a significant number of ruptures. This is particularly the case when sealing vessels, because a typical failure mode of vessels sealed with bipolar electrosurgery is a leak or rupture in the wall of the vessel adjacent to the sealed area.
The RF current inherently flows through the tissue in a somewhat random or uncontrollable pattern depending on the point-to-point characteristics of the tissue and many other factors. As a consequence, uniform heating of the tissue is impossible to control. The non-homogeneous distribution of heat over the area to be fused causes the protein chains to denature and reconstitute in a variable and nonuniform manner. The nonuniform denaturation and reconstitution leads to fused tissue areas of variable, nonuniform and somewhat unpredictable strength.
Assessing when to stop the delivery of RF current during bipolar electrosurgical tissue fusion is difficult. Applying either too much or too little RF current leads to seals that are more likely to fail. The application of too much RF current creates an excessive amount of heat which drives chemical reactions that appear to oxidize or burn the tissue and change the nature of the protein chains, thereby diminishing their ability to reconstitute and create effective seals. Overly-heated tissue at the sealed area or adjacent to the sealed area increases the probability of a failure because the tissue has become brittle and lacks pliability due to excessive dehydration, thereby contributing to cracking and breaking. In contrast, prematurely stopping the delivery of RF current prevents an adequate amount of denaturing of the protein chains which, in turn, prevents an adequate amount of reconstitution of the proteins chains, thereby diminishing the strength of the seal.
Control systems have been developed to attempt to address the problem of applying too much or too little RF power during bipolar electrosurgical tissue fusion. Such control systems monitor some event associated with the application of electrical power to the tissue, typically the impedance. Monitoring the tissue impedance is based on an expectation that some change indicates the occurrence of appropriate sealing conditions. However, it is believed that no reliable relationship exists between tissue impedance and the formation of a consistently reliable seal.
Another problem with bipolar electrosurgical tissue fusion is that the alternating aspects of the RF electrical energy inherently results in less energy application per unit of time. The alternating aspects of the RF energy application is by nature a pulsed or alternating current (AC) energy application, as opposed to a continual energy application. The tissue must withstand relatively high voltages, but the amount of power transferred is not commensurate with the high voltage due to the pulsed or AC application of the RF current. The effect of the pulsed or alternating RF energy application is that more time is required to transfer an equivalent amount of energy compared to the transfer of energy delivered at a sustained peak value. The typical maximum power delivery with a widely used RF tissue fusion device is approximately 115 to 350 Watts per square inch (18-56 W/cm2).
Electrothermal instruments have also been used for tissue fusion. Electrothermal instruments have heating elements within jaws that grip and compress the tissue. Electrical current is conducted through the heating elements to generate the heat that is applied to the compressed tissue. As with bipolar electrosurgery, previous electrothermal instruments have produced varying and inconsistent tissue fusion results, possibly as a result of an ineffective control system or control functionality based on misperceptions relating to tissue fusion physiology, including the perceived limitation of not heating the tissue above the 120° C. point where elastin protein chains denature. The prevalent view is to avoid elevating the temperature of the tissue beyond the 120° C. point where elastin protein chains denature, because it is believed that temperatures beyond that point are destructive to the proteins chains. Consequently, all presently known tissue fusion technologies attempt to limit the tissue temperature to no more than approximately 120° C., and many tissue fusion technologies limit the temperature of the tissue to approximately 100° C. to avoid creating steam.
Although the principal concern of tissue fusion is creating reliable seals that hold on a long-term basis, another very important practical consideration is an ability to create seals quickly. A typical surgical procedure will involve sealing many blood vessels at the surgical site. The typical time required by known electrosurgical tissue sealing devices to create a single seal is about 5-12 seconds. A considerable amount of time is therefore consumed in making each seal. Considering that a typical surgical procedure may require sealing scores of vessels, a considerable amount of the total procedure time is consumed by vessel sealing.
Moreover, because of concern about the reliability of the vessel seals, the typical practice is to create two sequential seals at each severed end of the vessel. The theory is that if the first or upstream seal fails, the second or downstream seal becomes a redundant backup to prevent fluid leakage. The time to create the primary and backup seals is more than twice the amount of time required to create a single seal when the time for repositioning and observing the quality of each seal is taken into account. Further still, double seals must be made at both ends of each severed vessel. Thus, a considerable amount of time is consumed during the surgical procedure by sealing vessels. The time consumed by sealing vessels extends the time required to accomplish the entire surgical procedure, or alternatively, detracts from the time available to accomplish other activities during the surgical procedure.